1. Field of the Invention
The present invention relates to a laser irradiation apparatus and a method for manufacturing a semiconductor device by using the laser irradiation apparatus.
2. Related Art
In recent years, a technique to form a thin film transistor (hereinafter referred to as a TFT) over a substrate has made great progress, and application development to an active matrix display device has been advanced. Particularly, a TFT formed using a poly-crystalline semiconductor film is superior in field-effect mobility to a TFT formed using a conventional amorphous semiconductor film, and therefore high-speed operation becomes possible when the TFT is formed using the poly-crystalline semiconductor film. For this reason, a circuit for driving a pixel, which has been mounted by an external IC chip, can be formed integrally with the pixel over the same substrate by a TFT.
The poly-crystalline semiconductor film suitable for manufacturing a TFT is obtained by crystallizing an amorphous semiconductor film. To crystallize the amorphous semiconductor film, a laser annealing method is generally employed. The laser annealing is more preferable than a general thermal annealing that requires a temperature as high as 600° C. or more. This is because an inexpensive glass substrate, which is often employed as a substrate of a TFT, is inferior in heat resistance and is easy to change in shape due to heat. That is to say, the laser annealing has advantages that the processing time can be shortened to a large degree compared with another annealing method using radiation heat or conduction heat and that a semiconductor substrate or a semiconductor film over a substrate can be heated selectively and locally so that the substrate is hardly damaged thermally. Therefore, the laser annealing method is widely used to crystallize the amorphous semiconductor film formed over the glass substrate.
It is noted that the laser annealing method described herein includes the technique to recrystallize an amorphous layer or a damaged layer formed in the semiconductor substrate or the semiconductor film and the technique to crystallize an amorphous semiconductor film formed over a substrate. In addition, the technique to flatten or modify the surface of the semiconductor substrate or the semiconductor film is also included.
It is preferable to perform the laser annealing in such a way that a beam spot is shaped into a square having a length of several cm on a side or into a line having a length of 100 mm or more through an optical system and that the beam spot is moved relative to the irradiated surface (or a region irradiated by the laser beam is moved relative to in a surface of a processing object) because this method provides high productivity and is superior industrially. (For example, see Reference 1.) It is noted that the term of linear herein used does not mean a line in a strict sense but means a rectangle having a large aspect ratio (or an oblong). For example, the rectangle having an aspect ratio of 10 or more (preferable in the range of 100 to 10000) is referred to as the line. It is noted that the linear is still included in the rectangular.
(Reference 1) Japanese Patent Laid-Open No. H08-088196
Since the laser beam emitted from the laser oscillator generally has Gaussian intensity distribution in which the intensity of the laser beam is attenuated from the center toward the end portion, it is necessary to homogenize the intensity distribution of the laser beam on the irradiated surface in order to perform the homogeneous laser annealing. In recent years, in order to form a linear beam having a homogenous intensity distribution, a method is known in which a lens array is used to divide the laser beam in a predetermined direction and to combine the divided laser beams. However, in this method, adjustment of the optical system requires a large amount of time. Here, the optical adjustment has two modes; setting an optical system such as a lens and a mirror on a predetermined position to perform a laser annealing; and after maintenance of a laser oscillator, correcting a misalignment of a laser beam involved by a misalignment of a window to perform the laser annealing again. Hereinafter, the former and the latter are defined as “adjustment” and “readjustment” , respectively. The latter, that is “readjustment” , is explained as follows.
A linear beam generally used to perform the laser annealing to the semiconductor film has an extremely narrow width, which is 1 mm or less. In order to homogenize the intensity distribution of the linear beam having such a narrow width in a direction of its short side, it is necessary to superpose laser beams divided by the lens array with very high accuracy. When one of the optical elements is displaced even a little, the intensity distribution of the beam spot formed on the irradiated surface may vary. For this reason, very accurate readjustment of the optical system is required, which consumes a large amount of time. During the readjustment of the optical system, the laser irradiation apparatus cannot be used and the process using the laser irradiation apparatus stops. This causes the throughput to decrease.
The optical system is readjusted by various reasons. Particularly, the optical system is most frequently readjusted when the laser beam is displaced from the predetermined position due to the maintenance work inside the laser oscillator.
As one of the maintenance works causing the misalignment of the laser beam from the predetermined position, cleaning of a window in a gas laser oscillator is explained here. In this maintenance work, the window of the gas laser oscillator is removed once in order to clean and then the window is set again to the same place in the laser oscillator. In this case, it is difficult to set the window to the same place and the window may be displaced. As a result, the laser beam is displaced from the predetermined position. This misalignment of the laser beam is explained with reference to FIGS. 1A and 1B.
A laser oscillator 101a shown in FIG. 1A has a pair of resonator mirrors 102a and 106a, a pair of windows 103a and 105a, and O-rings 104a. A flat glass 107a is not rotated in any directions because the laser beam is in the predetermined position. The laser beam emitted from the laser oscillator 101a forms a beam spot having a homogenous intensity distribution on an irradiated surface 109a by transmitting through an optical system 108a. 
On the other hand, FIG. 1B shows a laser irradiation apparatus in which an incident position of the laser beam into the optical system is different from that shown in FIG. 1A. In FIG. 1B, since the window 105b is not set to the predetermined position, the laser beam is displaced from the predetermined position. This is because the O-ring for fixing the window is made of rubber When the stress applied to the O-ring is not homogeneous, the O-ring is not pressed homogeneously. Since it is a man that sets the window to the laser oscillator, it is impossible to apply the completely homogeneous stress to the whole O-ring, and therefore the O-ring is not pressed homogeneously. As a result, as shown in FIG. 1B, the window is tilted from its original position where the window is set before being removed.
It is noted that FIG. 1B uses the same laser oscillator as that shown in FIG. 1A. The laser oscillator 101b has a pair of resonator mirrors 102b and 106b, a pair of windows 103b and 105b, and O-rings 104b. A flat glass 107b, an optical system 108b, and an irradiated surface 109b in FIG. 1B correspond to the flat glass 107a, the optical system 108a, and the irradiated surface 109a in FIG 1A.
The optical systems 108a and 108b are designed and positioned so that beam spots having a homogeneous intensity distribution are formed on the irradiated surfaces 109a and 109b respectively when the laser beam is in the predetermined position. Therefore, when the laser beam is displaced from the predetermined position, the laser beam is incident into a different position in the optical system, and the intensity distribution of the beam spot formed on the irradiated surface may not be homogeneous. As a result, the annealing may not be performed homogeneously to the irradiated object. When the laser annealing is performed using a semiconductor film as the irradiated surface in such a way that this beam spot is scanned relative to the semiconductor film and when TFTs are manufactured using a crystalline semiconductor film obtained thus, the electrical characteristic varies between the TFTs, and moreover the reliability may be lowered. In view of these problems, it is necessary to correct the misalignment of the laser beam from the predetermined position and to keep the incident position of the laser beam into the optical system the same in order to obtain the homogeneous intensity distribution on the irradiated surface and in order to perform homogeneous annealing to the irradiated object.
In order to correct the misalignment of the laser beam from the predetermined position, a steering mirror has been used conventionally. With reference to FIGS. 10A and 10B, a general example is explained in which the steering mirror is used to correct the misalignment of the laser beam. In FIG. 10A, the misalignment of the laser beam emitted from a laser oscillator 1501 is corrected by moving the laser beam parallel using two steering mirrors.
To correct the misalignment of the laser beam, it is effective to readjust the position of the laser beam by steering mirrors 1502 and 1503 while confirming the predetermined position of the laser beam with the use of a CCD camera 1504. It is noted that since the CCD camera 1504 is used only to correct the misalignment, it is set so that it can be easily removed after correcting the misalignment. An optical system 1505 has a structure for homogenizing the intensity distribution of the laser beam and for shaping the laser beam into a linear beam on an irradiated surface 1506.
However, the method using the steering mirror has the following problem. The steering mirror can only control the position of the laser beam but also change a traveling direction thereof Therefore, when the laser beam propagates from the steering mirror 1502 to the steering mirror 1503, the traveling direction thereof may change. In FIG. 10A, the misalignment of the laser beam from the predetermined position is corrected by the steering mirrors 1502 and 1503. On the other hand, in FIG 10B, the traveling direction of the laser beam changes by the steering mirrors 1502 and 1503. In order to clarify this change of the traveling direction, the traveling direction of the laser beam in FIG. 10A is shown by a dotted line in FIG. 10B.
When the laser beam whose travel direction changes is incident into the optical system, the intensity distribution of the linear beam spot formed on the irradiated surface 1506 may vary. Even though the CCD camera 1504 is used, the change of the traveling direction of the laser beam cannot be detected.
Although the position of the laser beam observed by the CCD camera 1504 is the same in FIGS. 10A and 10B, it is understood that its traveling direction is different. Thus, the conventional technique in which two steering mirrors are used to correct the misalignment of the laser beam from the predetermined position has a problem to be solved.
Although the misalignment of the laser beam from the predetermined position can be corrected by readjusting the optical system, this method is not preferable because the readjustment requires high accuracy and consumes a large amount of time as described above.